Ben Barba has reportedly been interviewed as part of the NRL's salary cap investigation into the Broncos.

AAP: Dan Peled

Brisbane's star recruits Ben Barba and Martin Kennedy have reportedly been interviewed as part of the NRL salary cap investigation into the club.

News Corp reports at least six Broncos squad members have been summoned to interviews with the NRL over the past two weeks, including Brisbane's highest-paid recruits for 2014 - Barba and Kennedy.

The probe is believed to have hit a stumbling block because the integrity unit cannot persuade former Broncos football manager Andrew Gee to cooperate with them.

Barba and Kennedy defected to Brisbane from Canterbury and the Sydney Roosters respectively, on multimillion-dollar deals with negotiations including the prospect of third-party deals on top of their base salaries.

The NRL's inquiry was launched in late May and is believed to centre around the Broncos possibly guaranteeing third-party payments.

The report claimed a potential smoking gun is a $400,000 loan from the Leagues Club, a separate entity to the publicly listed NRL business, to Gee.

Internal records show the money was repaid shortly before Gee resigned in April.

It is understood the Australian Securities and Investments Commission (ASIC) has been contacted to assist inquiries into the Leagues Club and could have the power to interview Gee because he once served as a board member.

The task of proving any of the Leagues Club loan was used to guarantee unfulfilled third-party deals has become difficult without the assistance of Gee.

The report claims there is no suggestion the players or their agents had any knowledge of wrongdoing or did anything wrong themselves.